In 1966, JPL had proposed to NASA a "Grand Tour" of the outer planets. The lab's navigators had shown that if a long-lived spacecraft were launched in 1977, a once-in-176-years alignment of the planets would allow a single spacecraft to visit all four of the gas giants. The navigators planned on using each planet's gravity to accelerate the spacecraft toward the next target. To travel such vast distances, the spacecraft would be powered by radioisotope thermal generators.

NASA approved the Grand Tour in 1971, cancelled it in 1972, and then resurrected it in a shrunken format in 1973. The mission, called Mariner-Jupiter-Saturn 1977 (MJS '77, officially), was intended to visit only Jupiter and Saturn. JPL's project team, however, still wanted to carry out the Grand Tour and decided not to limit the spacecraft's capabilities to the approved three and one-half year Jupiter/Saturn mission. If the vehicles survived their Saturn encounters, one or both might be redirected to continue the tour.

MJS 1977 gained the name "Voyager" after JPL's John Casani became project manager in 1976. Casani thought MJS was a terrible name for a robotic explorer and advocated a more distinctive title.

The Voyager mission, launched in 1977, would become JPL's longest-lived ever, with both craft still operating and returning data as of mid-2006.